This invention relates to a continuous direct esterification process for use in production of poly(ethylene terephthalate) from ethylene glycol and terephthalic acid, and is more particularly concerned with recovery and reuse of ethylene glycol vaporized in the esterification reaction.
Direct esterification processes for use in production of poly(ethylene terephthalate) involve esterification of terephthalic acid with ethylene glycol, followed by ester condensation polymerization. Water formed in the reactions is vaporized and removed. In a continuous process, the terephthalic acid is mixed with an excess of ethylene glycol to form a slurry or paste which is then heated to form a low molecular weight reaction product. The reaction is carried out by mixing the slurry with low molecular weight reaction product which is heated to the required reaction temperature, as illustrated in Leybourne III U.S. Pat. No. 3,590,072, Lewis et al. U.S. Pat. No. 3,676,485, Balint et al. U.S. Pat. No. 3,697,579, and Chapman et al. U.S. Pat. No. 3,927,982. The latter three patents illustrate reactors having an external heater, with reaction product circulating from the reactor vessel through the heater and back to the reactor, with the slurry being fed into the circulating product prior to the heater. All or most of the esterification reaction then takes place during passage of the glycol and terephthalic acid through the heater, and the reactor vessel serves primarily for separation of vapor from the reaction product. The low molecular weight polyester produced is fed from the reactor to subsequent polymerization steps for forming high molecular weight polymer suitable for melt-spinning into polyester yarn in conventional manner.
The excess ethylene glycol used in the slurry feed is vaporized and removed from the reactor with vapors of water and small amounts of organic impurities formed in the reaction (e.g., acetaldehyde, 2-methyl-1,3-dioxolane and 1,4-dioxane). These vapors have been condensed and processed to recover the ethylene glycol for reuse. A process for purifying the ethylene glycol is disclosed in Pierson U.S. Pat. No. 3,367,847, but a simpler and less expensive method is desirable.
The reactor vapors will also contain vaporized or entrained reaction product. The Lewis et al. patent discloses use of a partial condenser above the reactor vessel, which is operated at a temperature low enough to condense "vaporized monomer" as a liquid but now low enough to condense the ethylene glycol. The patent also discloses (column 4, lines 65-73) that a distillation column can be used in place of the partial condenser if a greater reduction in loss of monomer is desired, but there is no reference to recovery of ethylene glycol from the off-vapors.
Removal of ester product from the vapors prior to recovery of vaporized ethylene glycol is important. At the temperatures used to separate ethylene glycol from more volatile organic impurities and water formed in the reaction, ester product will form deposits which cause fouling in the recovery operation.